Mobile communication has made considerable progress in recent years. Starting from a traditional circuit switched (CS) approach as e.g. pursued in the Global System of Mobile Communication (GSM), the technological evolution has made its way towards packet switched (PS) systems such as those operating for example on the basis of the Internet Protocol (IP). Examples of such communication systems are the Universal Mobile Telecommunication Standard (UMTS) also referred to as Third Generation (3G) telecommunication system, or a system known as IMS system (IP Multimedia System), or a network operating on the basis of the Session Initiation Protocol SIP. UMTS is not necessarily operating based on IP, it has a PS domain that is more powerful than GSM's PS domain and therefore it can support VoIP. UMTS provides also circuit switched services.
While in former systems, speech or voice was a main aspect of “media” as payload data to be conveyed via the communication system, nowadays all kind of media are focused on as payload, e.g. speech, music (audio), images (still images, i.e. pictures, as well as moving images, i.e. videos), executable program code, text data, and many other types or combinations of media types.
With the emerging development of new systems, those “old” and new systems coexist in parallel and interoperability is one aspect of the development. In particular in case of users (represented by their terminals, e.g. mobile stations MS in GSM or user equipment UE in UMTS and/or IMS) that may move and/or roam throughout the communication networks, the users expect seamingless services of high quality to be provided to them, irrespective of the current communication system in which they camp, i.e. irrespective whether the system is packet or circuit switched.
A call from/to a terminal represents a logical association between two endpoints, one of which being the terminal. These endpoints are often two terminals, but also the endpoints are terminal considered and the network. In the latter scenario, one side my be PS and one CS for an terminal to terminal connection. In a CS domain, the call relies on a bearer connection between the endpoints. Stated in other words, this means that service and almost physical channel description are combined to one CS service, while in PS with, e.g. SIP, the service control is to a large extent independent from how the bearer is provided. In a PS domain, the call relies on a session established between the endpoints. The PS, CS, or any call comprises signaling and payload (media) delivered, and thus a signaling part is established as well as a media part. Channel is rather physical and does not fit well for PS as both components of a PS call may be carried on the same physical channel. Each of those channels or parts, respectively, thus may in principle rely on PS or CS access domain. A domain as used herein denotes the communication principle applied, i.e. PS or CS, or more generally an underlying radio access technology RAT and not the geographical location. That is, within a single geographical location or area, both PS and CS domains may coexist. Inter domain HO is PS-CS handover and may be performed also without RAT change.
It is to be noted that the examples given in conjunction with the present invention are not intended to be limiting in any way. Rather, specific terminology as used in some passages in this document is adopted as an example only to simplify the description of the present invention and to illustrate a particular case to which the present invention may advantageously be applicable. This, however, is not intended to exclude any other field of application for the present invention.
A user equipment UE may be communicating in an area that can be denoted as cell in a PS domain of a network. When the UE is moving or roaming through the network, it may experience coverage problems which necessitate a change of a serving entity and specifically the need to change to an area that can continue to serve the communication by the CS domain only. In a PS domain, a serving entity is e.g. represented by a SGSN (Serving GPRS Support Node, GPRS=General Packet Radio Service). In a CS domain, a serving entity is e.g. represented by a MSC (Mobile Services Switching Center). Such change of serving entity is known as handover HO. In networks/areas in which PS domains as well as CS domains coexist, hitherto a handover (HO) had to be performed within the same domain so as to provide continuous service (intra-domain HO). With the increasing demand of users and the increasing roaming behavior of users, it is desirable that users may also be enabled to roam from a PS domain to a CS domain to thereby overcome the problem that there is no full coverage by both domains, e.g. the rollout of the PS coverage takes some time so that in the rollout area PS services can be provided. But when the UE leaves these areas it has to be served by more widely deployed CS services. Therefore the inter domain handover has to be accomplished. Under the above previous assumption, the service would have to be interrupted. It is therefore a need to provide for enablement of HO also between the domains, i.e. to provide for so-called inter-domain HO. One type of inter-domain HO is referred to as single radio voice call continuity, SR-VCC.
Work is ongoing to define single radio voice call continuity (SR-VCC) procedures in a further release of standards. SR-VCC is studied extensively and will be defined in a forthcoming standard. SR-VCC enables the UE to perform an inter-domain handover from PS domain (such as from a LTE (long term evolution) network “cell”) to CS domain (such as to a GSM network cell), and vice versa. HO is performed such that signals from potential target neighbouring cells are measured by the roaming terminal in terms of the respective signal strength, the target cells are evaluated for being suitable, and finally a suitable target cell is selected and HO is initiated and performed thereto. As a result of the HO procedure, the terminal is then experiencing the services that are served by the new serving entity.
When moving the development further in such a direction towards inter-domain HO's, various problems arise that need to be solved in order to achieve the goal.